1. Field of the Invention
This invention relates to a particle measurement apparatus, and more particularly to a particle measurement apparatus in which a laser beam is projected at a sample in a measurement zone and light scattered by particles in the sample is evaluated to thereby measure properties of particles in the sample.
2. Description of the Prior Art
Optical measurement apparatuses in conventional use include those employing scattered light to measure particles in a liquid or gaseous medium in which the concentration of the particles in the medium is very low, so that only one or two particles traverse the measurement zone at one time. The object of such apparatuses is to measure particle size and other such properties by using a sensor to evaluate the scattering of the light projected at the liquid or gas concerned. Conventionally, these scattered light type particle measurement apparatuses usually employ a laser light source. A laser light source provides a light beam that can be converged to a fine beam at the focal point. In such an arrangement, the energy density can be increased at the location where the illuminating beam is focussed, thus enabling the measurement of particles as small as 0.1 micrometers to (see, for example U.S. Pat. No. 4,830,494).
With this type of particle measurement apparatus that utilizes a laser light source, the laser beam is projected at a measurement zone through which the gaseous or liquid sample flows, the scattered light is measured at a predetermined angle relative to the direction of incidence of the laser beam and converted to electrical signal pulses by a photoelectric converter. The output signals are then discriminated in terms of a pulse height, and counted by an internal counter corresponding to a predetermined size to measure the number of the particles that have the predetermined size.
However, the intensity of a laser beam produced by a single mode laser has a Gaussian distribution in a plane, normal to the beam axis. Thus, if the center of light intensity is I.sub.O, the relationship between the intensity I and the distance from the center r is expressed by EQU I=I.sub.O e.sup.-2(r/rO).sup.2
(r.sub.O being r when I=I.sub.O e.sup.-2). Therefore, the intensity of scattered light from particles passing through the laser beam will vary according to the position of the particle. Thus, the intensity of the scattered light from the particles will not be the same even when the particles concerned are the same size. Instead, scattered light from a particle passing through a region of high light intensity will have a high intensity and scattered light from a particle passing through a region of low light intensity will have a low intensity, resulting in the same sized particles being counted as particles of different sizes.
To avoid this, as explained below, there is a measurement method comprising the steps of arranging a plurality of light receiving systems at different angles with respect to the axis of the laser beam, finding the ratios of the scattered light intensities obtained from the light receiving systems, and determining the particle size on the basis of the relationship between the scattered light intensity ratios and particle size.
In such an arrangement, there are provided a first light receiving lens that receives scattered light at a forward angle of 6 degrees to the laser beam axis and a second light receiving lens that receives scattered light at a forward angle of 3 degrees to the laser beam axis. The scattered light in each case is detected by a photosensor and converted to electrical signals which are amplified by an amplifier circuit. A divider is used to obtain the ratio of the scattered light intensities. In accordance with Mie's scattering theory, as described below, there is a predetermined relationship between particle size and the ratio, of the scattered light intensities. Thus, the particle size can be determined based on this predetermined relationship.
However, when such an arrangement is used that includes two light receiving lenses to detect scattered light at different angles, there is a discrepancy between the fields of view of the lenses. As a result, a scattered light signal from a particle may be received from one of the light receiving lenses but not from the other. This would not present a problem if there were a complete absence of a scattered light signal from one of the light receiving systems, but if even a small amount of noise should intrude into the light receiving system, there is a risk that the ratio of the noise to the signal from one of the light receiving systems may be wrongly interpreted as being the ratio of scattered light intensities produced by a particle.
Also, when a signal is received that exceeds the dynamic range of the light receiving systems, there is a risk that the ratio of the saturated values may be included in the particle count.